The present invention relates generally to integrated circuits, and more particularly, to techniques and circuits for improving noise margins on on-chip terminated I/O busses and reducing power supply droop and ground bounce oscillations.
One of the causes of reduced, or variable, termination voltages is a change in the amount of current drawn from the termination voltage supply lines. The changes in the amount of current drawn excites oscillations in the inherent inductances in the termination voltage lines. These inherent inductances include inductaces from the package leads and bond wires. The frequency of these oscillations depends upon a number of factors that vary from package-to-package and chip-to-chip. However, on a high-performance I/O (input/output) bus, the frequency of operation may be much greater than the frequency of the oscillations on termination voltage lines. Accordingly, it is important that the I/O circuits on these busses be designed to operate over a range of termination voltages.
If circuits are not designed to operate over a range of termination voltages, the lines on the bus may not meet their switch times or noise margin requirements and the operating frequency of the bus may have to be lowered. Thus, to meet frequency goals, the termination voltage may be increased to obtain minimum acceptable operating conditions. This increased termination voltage increases the integrated circuit""s power dissipation. Increased power dissipation can increase the cost of several components of a system including the integrated circuit packaging, heat sink, and the system power supply. Furthermore, increasing the termination voltage tends to decrease the operating lifetime of the part thereby increasing the cost of system maintenance and amortized cost.
Accordingly, there is a need in the art for an apparatus and method that reduces the changes in the amount of current drawn on a bus termination voltage supply.
An embodiment of the invention reduces the changes, or variability, in the amount of current drawn from the termination voltage supply of an I/O bus. This, in turn, reduces the range of voltages over which a termination voltage may vary. It is well adapted for fabrication on integrated circuits and can be particularly effective when used on wide, parallel, high-speed I/O busses.
Instances of an embodiment of the invention are connected to the wires of an on-chip terminated I/O bus. Each instance monitors the wire that it is connected to. If the wire has been pulled low by any device on the bus, the circuit does nothing. If, however, the wire was not pulled low, then the invention shunts current from the termination voltage supply to ground. The turn on and turn off rates for this current shunt are matched to the ramps of current through the termination resistor of the bus. This makes the variability in current drawn from the termination voltage supply less data dependent. Making the current drawn from the termination voltage supply less data dependant reduces the magnitude of the inductive oscillations on the termination voltage which reduces the range of termination voltages over which the bus must be designed to operate.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.